As pointed out in commonly owned U.S. Pat. No. 4,385,883, issued May 31, 1983 to Wilhelm Hanslik, it has already been proposed to equip such extruders with a device comprising several dished and coaxially superposed heating plates whose bottoms are provided with relatively offset apertures and are individually scraped by bladed wheels rotatable about their common axis to let the material successively pass through the several heating stages constituted by these plates. Similar multistage heaters are known in the field of metallurgy for the roasting of sulfitic ores with or without chlorination.
Such an assembly of stacked heating plates has a relatively limited throughput rate and must therefore be of large dimensions to handle a substantial quantity of material to be treated. If one stage becomes defective, the entire stack must be disassembled to enable its replacement. Since the material passes through the stack only once, heating to high temperatures requires a large number of stages.
The Hanslik patent discloses and claims a pretreatment stage located in an extruder housing between an inlet for polymeric bulk material and a feedscrew for plasticising and masticating that material prior to discharging it under pressure to a shaping die. The pretreatment stage includes two juxtaposed or coaxially interfitted augers designed to circulate the incoming material from the inlet to a remote location and to deliver it from there to the entrance of a channel leading to a main housing part containing the feedscrew, this entrance lying in the vicinity of the inlet and communicating therewith via a passage enabling the recirculation of the material through a closed loop by the two augers upon obstruction of the channel.
The assembly of this prior patent, in which the pretreatment stage and the main housing part are surrounded by external heating means, operates very satisfactority when used at or near its throughput capacity. With smaller production rates, however, that device becomes less efficient so that its utilization in such cases is not very economical.